In commit 10154dbded ("udp: Allow GSO transmit from devices with no
checksum offload") we have intentionally allowed UDP GSO packets marked
CHECKSUM_NONE to pass to the GSO stack, so that they can be segmented and
checksummed by a software fallback when the egress device lacks these
features.
What was not taken into consideration is that a CHECKSUM_NONE skb can be
handed over to the GSO stack also when the egress device advertises the
tx-udp-segmentation / NETIF_F_GSO_UDP_L4 feature.
This will happen when there are IPv6 extension headers present, which we
check for in __ip6_append_data(). Syzbot has discovered this scenario,
producing a warning as below:
ip6tnl0: caps=(0x00000006401d7869, 0x00000006401d7869)
WARNING: CPU: 0 PID: 5112 at net/core/dev.c:3293 skb_warn_bad_offload+0x166/0x1a0 net/core/dev.c:3291
Modules linked in:
CPU: 0 PID: 5112 Comm: syz-executor391 Not tainted 6.10.0-rc7-syzkaller-01603-g80ab5445da62 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/07/2024
RIP: 0010:skb_warn_bad_offload+0x166/0x1a0 net/core/dev.c:3291
[...]
Call Trace:
<TASK>
__skb_gso_segment+0x3be/0x4c0 net/core/gso.c:127
skb_gso_segment include/net/gso.h:83 [inline]
validate_xmit_skb+0x585/0x1120 net/core/dev.c:3661
__dev_queue_xmit+0x17a4/0x3e90 net/core/dev.c:4415
neigh_output include/net/neighbour.h:542 [inline]
ip6_finish_output2+0xffa/0x1680 net/ipv6/ip6_output.c:137
ip6_finish_output+0x41e/0x810 net/ipv6/ip6_output.c:222
ip6_send_skb+0x112/0x230 net/ipv6/ip6_output.c:1958
udp_v6_send_skb+0xbf5/0x1870 net/ipv6/udp.c:1292
udpv6_sendmsg+0x23b3/0x3270 net/ipv6/udp.c:1588
sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg+0xef/0x270 net/socket.c:745
____sys_sendmsg+0x525/0x7d0 net/socket.c:2585
___sys_sendmsg net/socket.c:2639 [inline]
__sys_sendmmsg+0x3b2/0x740 net/socket.c:2725
__do_sys_sendmmsg net/socket.c:2754 [inline]
__se_sys_sendmmsg net/socket.c:2751 [inline]
__x64_sys_sendmmsg+0xa0/0xb0 net/socket.c:2751
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
[...]
</TASK>
We are hitting the bad offload warning because when an egress device is
capable of handling segmentation offload requested by
skb_shinfo(skb)->gso_type, the chain of gso_segment callbacks won't produce
any segment skbs and return NULL. See the skb_gso_ok() branch in
{__udp,tcp,sctp}_gso_segment helpers.
To fix it, force a fallback to software USO when processing a packet with
IPv6 extension headers, since we don't know if these can checksummed by
all devices which offer USO.
Fixes: 10154dbded ("udp: Allow GSO transmit from devices with no checksum offload")
Reported-by: syzbot+e15b7e15b8a751a91d9a@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000e1609a061d5330ce@google.com/
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://patch.msgid.link/20240808-udp-gso-egress-from-tunnel-v4-2-f5c5b4149ab9@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
30b03f2a05